With approximately 80% of voice mobile cellular users located indoors, cellular networks and systems such as Universal Mobile Telecommunications System (UMTS) aim to provide consistent indoor coverage. Around 95% of data mobile cellular users reside inside buildings, typically using High-Speed Downlink Packet Access (HSDPA). HSDPA is a high speed data service that can be deployed on top of the UMTS layer. HSDPA performance inside buildings especially poses a coverage challenge to traditional strategies.
Buildings create high penetration losses, high power load per user, and drain the capacity of the UMTS. Inside buildings there is very often a lack of single cell dominance, resulting in larger soft handover zones. Due mainly to the lack of isolation between several serving cells, HSDPA performance is limited. UMTS/HSDPA is a noise sensitive system, thus HSDPA performance, namely data speed, is dependent on the signal to noise ratio (SNR) and the signal to interference ratio (SIR). Improving HSDPA performance depends on the geometry factor, G, given as:
  G  =                    Received        ⁢                                  ⁢        Signal                    Thermal        ⁢                                  ⁢        Noise              +    Interference  
Interference results from network traffic and from other cells. A good geometry factor which will produce higher speeds requires good cell isolation between the cells. There are no soft handovers in HSDPA and the cells operate on the same channel. Thus, neighbor cells are potential noise sources to the HSDPA user.
Traditionally, microcells are often deployed in high use areas in city centers. Where a macrocell does not sufficiently cover an intended area, a microcell is deployed to provide coverage to that area. Microcells deployed in high traffic urban areas typically provide good deep indoor coverage and boost capacity by a low frequency and scrambling code reuse factor. Each microcell antenna radiates a separate cell with typical inter-site distances of about 300-500 meters. However, standard deployment gives poor isolation between neighboring cells. Soft handover zones typically cover 30-40% of the area in a traditional microcell roll-out. These soft handover zones load the backhaul and impact the capacity of the network. Also, the SNR between neighboring HSDPA cells will degrade data throughput in overlapping areas. Other microcell difficulties include dropped calls, high power load on the UMTS, and a high production cost for the operator.
Another communication service protocol is Orthogonal Frequency-Division Multiple Access (OFDMA) which is a digital modulation scheme where multiple access is achieved by assigning subsets of subcarriers to individual users, allowing simultaneous low data rate transmission from several users. Other protocols which may be used include, but are not limited to, Worldwide Interoperability for Microwave Access (WiMAX) and Long Term Evolution (LTE). These protocols can be used with multiple-input multiple-output (MIMO) networks. In typical MIMO networks, multiple antennas are closely spaced to each other. However, consistent signal coverage is not achieved in a closely spaced MIMO network, resulting in more frequent soft handovers (except for WiMAX and LTE systems which do not have soft handovers), lower average data rate, lower battery quality, and lower voice quality.